This invention relates to analyzers for detecting levels of glucose in liquids such as whole blood, and, more particularly, to an improved membrane and method of making the same which can be used with such glucose analyzers.
Glucose analyzers have been developed which perform relatively rapid sampling of whole blood from a patient being monitored to analyze the same for glucose level. Such devices periodically withdraw such blood samples in vivo and allow attending personnel in hospitals to have a fairly current reading of the glucose levels of patients.
The purpose of such analyzers is to keep a constant track of such levels and are particularly useful with diabetic patients when transient glucose level spikes may give inadequate, or in some cases false indications of symptoms of certain patients, thus continuous monitoring can detect and qualify such false indications.
In addition, more frequent determinations are needed for glucose level when a diabetic patient is acutely ill, undergoing major surgery, childbirth or suffering from severe ketoacidosis.
The need for such continuously monitoring instruments has brought some to the market, but those face problems regarding precision, clotting and drift in blood sampling and detection systems and non-linearity of signal output.
A relatively new instrument has been proposed based on an immobilized enzyme electrode which operates in a rate detection mode. The enzyme electrode comprises a membrane having glucose oxidase bonded to a teflon wafer. One side of the membrane contacts the withdrawn whole blood while the other side is in contact with a polarographic electrode for detecting oxygen.
The instrument operates on the principal that, in the presence of glucose, oxygen tension rapidly falls as oxygen is consumed at the membrane end of the electrode. By observing and recording the rate of fall in oxygen tension, one can determine the glucose concentration due to the equation and the stoichiometric relationship between oxygen and glucose as follows: ##STR1##
The immobilized enzyme membrane thus reacts, through the presence of glucose oxidase bonded to its surface, glucose and oxygen, and as the reaction proceeds, the polarographic electrode contacts the other surface of the membrane and gives off a signal as to the rate that the oxygen tension decreases. The membrane itself, being hydrophobic, prevents other liquids in the whole blood from passing through.
Such an instrument can achieve good specificity for oxygen since O.sub.2 is the only electro-active substance in blood that will diffuse through Teflon, and specificity for glucose is also excellent due to the known specificity of glucose oxidase for glucose.
As can be readily seen, the membrane is an important component of such system, having an immobilized enzyme bonded to its surface in a manner to create a stable membrane to achieve reliable, repeatable results.
Such membranes have been known to use three particular reagents, paraformaldehyde, bovine serum albumin and glucose oxidase applied to a teflon base wafer having an etched surface, however, the particular proportions of the three reagents resulted in a membrane having relatively low shelf life, low activity and experienced problems in cracking of the enzyme coating during use.
In particular, a membrane was known by the inventions hereof which was produced by using solutions of 20% bovine serum albumin, 10% glucose oxidase and 4% paraformaldehyde in the respective proportions of 1:2:1 by volume. When tested in a commercial Beckman glucose analyzer such membranes showed activity evidenced by a reading of 61% of the gravimetric standard concentration. Their activity dropped about 23% in ten days, showing relatively poor shelf life. At the end of two weeks most of the membranes tested evidenced about 50% of their original activity.